The invention relates to a sample&hold amplifier (SHA) with current source charger, particularly for systems requiring a fast and highly accurate analog storage of a instantaneous value of a input voltage.
The conventional SHAs require two operational amplifiers (OA), a capacitor, a switch and a control circuit. The capacitor is coupled to ground or the output of the SHA. A diode bridge as the switch and a charging amplifier are also used in order to reduce the acquisition time.
The conventional SHAs have a number of major disadvantages. At least one of the OAs is overdriven while closing the switch which also conducts a high and not exactly determined short circuit current. In high speed applications a thermal overload of the SHA can result which impairs its accuracy due to an increased temperature drift The employment of the nonlinear charging amplifier demands matching of the individual diodes. The invention is intended to provide an SHA including an 0A always operating in a normal mode, whereby the charging and discharging currents of the capacitor are accurately predetermined. An SHA according to the present invention includes comparator means having a pair of inputs with the input signal coupled to one input, integrator means having an input and an output coupled to the other comparator means input, and control means for determining a charging current for the integrator means input in response to the input signal via the comparator means and the integrator means output via the comparator means.
The comparator solves the problem of the OA overdrive and the integrator enforces a most desirable operation of an OA. The output voltage of the integrator and the input voltage of the SHA are applied to the inputs of the comparator B having the output K coupled to a control circuit.